The invention relates to a hollow cathode system, a device and a method for the plasma-assisted treatment of substrates, in particular a coating, a removal of substrate surfaces or an activation of the substrate surfaces for a subsequent treatment.
For the plasma-assisted deposition of thin films on substrates of a large surface area, usually large-area, plane-parallel electrode systems with electrode areas of a size corresponding approximately to that of the substrate area are used. However, these have the disadvantage that only low depositing rates can be achieved with them, and they are restricted with regard to the use of excitation frequencies in the very high-frequency range (VHF) by the generation of standing waves.
Customary methods for the activation of vapors with thermal cracking stages in vaporizers have decisive disadvantages: temperatures of 1200° C. and more are required for the generation of atomic Se from Se molecules evaporating from the evaporation material, which is desired for certain processes because of its relatively high reactivity.
Linear planar electrodes with areas of a width that is significantly less than their length, with or without being arranged in series, are described in DE 102 01 992 B4. The substrate is in this case generally located on a grounded substrate support, which is moved linearly, parallel to the width of the electrode. Also in such an arrangement, only low plasma densities are achieved, which leads to only low working speeds and consequently low productivity. Moreover, very small electrode spacings are necessary in such an arrangement for certain applications, typically lying below 10 mm. This leads to a high level of mechanical production complexity. Such an arrangement also often leads to a high current flow over the grounded substrate, the way in which the substrate contact is realized having a significant influence on the process of working the substrate.
In order to increase significantly the plasma density generated by a plasma source, hollow cathode arrangements are therefore used. A typical application area for this is the plasma-assisted working of surfaces. Gas discharges by means of hollow cathodes can be generated with DC voltage, with high-frequency pulsed DC voltage or else with high-frequency AC voltages. Leonhardt et al., Vakuum in Forschung and Praxis [vacuum in research and practice] 1995, 1, 17, gives here as a rule of thumb for the pressure p in dependence on the distance between the areas d forming the hollow cathodes: p×d=1 mbar cm. The greater the distance d, the lower therefore the pressure p at which the discharge ignites or working is performed.
Therefore, on account of the already many years of use and the very varied areas of use, there are a large number of configurations for hollow cathode arrangements, the most relevant of which for the invention are to be explained below.
Various hollow cathode systems with different electrode designs are described in DE 195 05 268 A1. They are characterized by separate, individual or in-series, linear hollow cathodes or an array of hollow cathodes surrounding the substrate or by a plane-parallel electrode system with a mesh-like partially transparent electrode. The feeding in of gas takes place in this case in a defined manner through regions of the hollow cathodes. Because of inhomogeneous igniting conditions, however, the range of operating parameters must be restricted here, which in turn is disadvantageous for an application.
Takeuchi et al., Thin Solid Films 390 (2001) 217, describes conductor electrodes with excitation frequencies of 60 to 80 MHz, which consist of an array of linear hollow cathodes. These are formed by a series arrangement of in each case two rod-like electrode elements. The feeding in of reaction gas takes place either through openings in the rod electrodes or through a gas showerhead, which is arranged on the side facing away from the substrate electrode, so that the conductor electrode lies between the substrate electrodes and the gas showerhead. Also disadvantageous in the case of this hollow cathode system are the plasma formation that is inhomogeneous for steady-state PECVD, with tolerances of the properties of the layers of over 10%, and also the complicated coupling in of power.
Multihole-array electrodes for PECVD of amorphous and multicrystalline silicon are described by Niikura et al., Proc. 19th EUPVSEC, Paris 2004, 1637, and Thin Solid Films 457 (2004) 84. These are electrodes with a gas showerhead that are coupled to the HF/VHF power supply, the gas outlet openings having been widened into hole-like hollow cathodes. However, the production of the electrodes in this case involves a high level of mechanical complexity.
DE 10 2010 030 608 B4 discloses a device for the plasma-assisted treatment of substrates with a hollow cathode, which is arranged on the electrode area facing the reaction space and is formed as an uninterrupted, grating- or meander-like channel in the electrode surface. In this case, a gas inlet system and a gas removal system are used, arranged as linear systems on the longitudinal sides of the hollow cathode electrodes. Consequently, both the supply of reaction gas and the discharge of residual gas take place by way of these linear systems, it being possible for the supply to be located on one longitudinal side of the electrodes and the discharge to be located on the other longitudinal side of the electrodes. In the case of this device it is disadvantageous that a feeding in of gas by means of cost-intensive gas distributor systems and gas inlet systems, possibly coupled with a relative movement between the gas inlet and the substrate, is required. The gas distribution requires an appreciable pressure difference between the gas inlet in the gas distributor and the reaction space with the substrate.
The object of the invention is therefore to develop a cathode, a device and a method for the plasma-assisted treatment of substrates of a large surface area with high treatment rates, in the case of which a homogeneous treatment of substrates of a large surface area can be achieved with high plasma stability over the entire electrode region, and an increase in the plasma density can be achieved by an excitation of the plasma with very high frequencies, and cost-intensive gas distributor systems can be avoided.